The basic way we know that God exists is by obedient to the promptings of our conscience. Our upbring is a factor in how our conscience has been trained. It can be trained not to believe that there is a God or a soul. Children cannot be blamed for that. If their parents are not believers, it is likely that the child will not believe. But children grow up and so are more responsible for their own actions. Does their conscience tell them that killing babies in the womb, murdering innocent people, cheating in order to get rich, molesting others in order to get pleasure are matters of indifference or are they matters that cry to the heavens for justice. It is at this point that the decision has to be made whether God does or does not exist. If he doesn't, then all deeds are allowed. If he does, there will be judgment in eternity. Fear of the Lord is the beginning of wisdom.